Multi-carrier broadband techniques, such as orthogonal frequency division multiplexing (OFDM), offer the possibility of increasing data rates and throughputs in wireless communications. As is well known in the art, these multi-carrier broadband techniques can provide wideband data rate performance in difficult channels by slicing the spectrum into “chunks” for transmission, optimizing the chunks, and reassembling them at the receiver. Due to the relatively broad frequency spectrum utilized, the channel quality can vary significantly among the multiple frequency carriers.
FIG. 1 graphically illustrates an example 11 of how the channel quality can vary across the occupied spectrum. The channel quality is represented in the FIG. 1 example in terms of SINR (signal-to-interference-and-noise-ratio). More specifically, each “chunk index” value in FIG. 1 corresponds to a group (or chunk) of carrier frequencies, and the curve 11 represents the SINR of the chunks. The carriers within a given chunk are conventionally assumed to have frequency responses that are similar enough to one another to assume that the same modulation and coding scheme (MCS) can be used for each carrier of the group.
The type of wide variation in channel quality illustrated at 11 in FIG. 1 can pose difficulties in the process of selecting a proper MCS, because the selection of the MCS is related closely to the channel quality that is anticipated for the transmission. The MCS selection process, often referred to generally as link adaptation, can be particularly challenging when transmitting information in small blocks, which is common in voice over IP (VOIP) services and several other types of real-time and streaming multimedia services.
In some prior art solutions, the receiver computes an average of the channel quality across the entire utilized spectrum, and feeds this average back to the transmitter. The average SINR shown at 12 in FIG. 1 is an example of such an average channel quality. The average channel quality can be useful if the information bits to be transmitted can be assigned to occupy a significant portion of the utilized spectrum. As examples, average channel quality can be useful for transmissions that can be assigned to occupy all of the chunks of the spectrum, or to occupy chunks distributed across the complete spectrum (e.g., all chunks with an odd chunk index).
However, the actual channel quality of a particular carrier or chunk can vary significantly from the calculated average channel quality. This can be seen in the example of FIG. 1 by comparing the actual channel quality 11 to the average channel quality 12. The deviation of actual channel quality relative to average channel quality can significantly compromise the link adaptation for small information blocks that are assigned to a relatively small portion of the spectrum. Under these circumstances, an MCS choice that is based only on the average channel quality can result in an undesirably high block error rate (BLER), which can lead to increases in latency and system load.
In other prior art solutions, the receiver measures the channel quality of every carrier across the entire spectrum, and feeds this information back to the transmitter for use in link adaptation. Although this approach permits acceptable link adaptation for transmissions of both large and small blocks of information, it typically requires an undesirably large portion of the available radio resources.
It is desirable in view of the foregoing to provide for effective link adaptation with all sizes of transmitted data blocks, while avoiding the aforementioned difficulties associated with prior art solutions.